Embodiments of the invention relate to sight glasses for indicating the level of a fluid within a temperature control unit or other device having a fluid reservoir.
Photolithographic processes typically use a device called a xe2x80x9cstepperxe2x80x9d for projecting a pattern onto a semiconductor wafer. It is important to maintain the projection lens of the stepper at a constant temperature to prevent damage to the lens and to provide accurate projection of the pattern. Temperature regulation is achieved by a temperature control unit (TCU). Examples of current commercially available TCUs are the 900/500/300 series of temperature control devices produced by ASM Lithography.
FIG. 1 provides a generic illustration of a stepper and TCU. The temperature control unit 100 includes a reservoir 102 that provides a cooling fluid to a stepper 104 through a heat exchanger 106 and receives heated fluid back from the stepper 104. To assist the operator in determining the level of fluid in the reservoir 102, the TCU also includes a sight glass 108 that is in communication with the reservoir 102. In this arrangement, fluid fills the sight glass 108 to the same level as fluid within the reservoir 102, allowing the level of fluid in the reservoir 102 to be judged by the level of fluid in the sight glass 108.
The TCU further includes a fluid level sensor 110 that monitors the level of fluid in the reservoir 102 and provides a signal to the stepper 104 when fluid falls below a minimum level that is required to maintain a stable operating temperature. For proper operation of the stepper 104 it is essential to maintain the amount of fluid in the reservoir 102 within an operating range, because when the fluid falls below the required minimum level the stepper automatically shuts down, and restarting and stabilizing the stepper can take eight to twelve hours before a stable operating condition is achieved.
One disadvantage of conventional sight glasses generally is that their usefulness tends to be degraded over time as deposits from the fluid within the sight glass obscure their clarity. This makes it progressively more difficult to read the fluid level within the sight glass. As a result, a low fluid level condition may be overlooked because it cannot be viewed without close examination, leading to stepper shutdown.
A further disadvantage of sight glasses in some TCUs is that the sight glass does not cover the full filling range of the reservoir. FIG. 2 shows an example of a sight glass of a conventional TCU. In this configuration, a sight glass 200 comprises part of a conduit 202 that is in communication with a reservoir (not shown). The conduit further comprises upper and lower portions 204, 206 that are not transparent or are blocked from view by other structures of the TCU. The maximum fill level 208 of the reservoir is within the upper portion 204 of the conduit, and as a result, it is not possible for an operator to judge whether a fluid level that is above the range of the sight glass is approaching the maximum fill level. Nevertheless, the operator is motivated to fill the reservoir as much as possible to help ensure that the reservoir does not fall below the minimum level and initiate stepper shut down. As a result, an operator may attempt to fill the reservoir beyond the level that is visible in the sight glass, but will be unable to know when to stop filling to avoid overflow. This creates a dangerous condition since overflow can result in damage to equipment and dangerous conditions such as electrical short circuit.
Embodiments of the invention provide solutions to problems of the conventional sight glasses and conventional temperature control units.
In accordance with a preferred embodiment, an apparatus for indicating a fluid level in a reservoir includes a conduit for communication with a reservoir and including a sight glass. A buoyant primary float is provided within the conduit, and a stop is provided within the conduit for engaging the primary float to prevent travel of the primary float beyond an upper portion of the sight glass. A buoyant secondary float in the conduit is moveable together with the primary float when the primary float is not engaged by the stop, and that is moveable separately from the primary float beyond the upper portion of the sight glass the when the primary float is engaged by the stop. The secondary float preferable includes an elongate portion that passes through an aperture in the primary float, and a buoyant upper portion that is located above the primary float and is larger than the aperture in the primary float. The secondary float also preferably includes a lower portion that is located below the primary float and is larger than the aperture in the primary float. The length of the elongate member between the upper and lower portions may be provided to correspond to a distance between a lower surface of the primary float when engaged by the stop, and a predetermined fluid level in the conduit such as the maximum fill level of the reservoir. The elongate member may also include markings calibrated to indicated a remaining fluid capacity of the reservoir relative to a predetermined capacity.
In accordance with another preferred embodiment, a temperature control unit includes a sight glass with primary and secondary floats as previously described.